Inside Facebook reports that Facebook has made a few tweaks to help Page owners understand the reach of their Posts:

Reach is the total number of people who have seen a page post within the first 28 days since it was made. Reach is counted when a post is loaded and shown in News Feed. Beginning today, mobile distribution will count toward a page’s total reach. It was previously unclear that reach data was limited to desktop only, so many pages might have been reaching more users than they had thought. However, it seems Facebook will report reach as a combined total, rather than distinguishing between desktop and mobile. Last yearthe social network temporarily reported whether post Likes and comments came from mobile, but that doesn’t seem to be available from the Insights API anymore.

Facebook made another change this week that will affect reach total. To improve the efficiency of News Feed, Facebook will load fewer stories at a time. When a user scrolls down the feed, more stories will load. Because reach and impressions are counted upon load, the metric previously included impressions that users might not have actually seen. Now insights should more accurately reflect what users have viewed, though it should not affect how many fans will see a page’s post in their feed.